Delfin
Grand Vizier
- Joined
- Jan 20, 2010
- Messages
- 3,822
For the battery obsessed, like me, the following might be interesting as it illustrates why LiFePO4 batteries are so different from Pb chemistry in terms of their charging behavior. The charge cycle below occurred after a typical usage cycle for us of around 300 amps. This would discharge the 600 amp bank around 50%. When re-charging our prior AGM bank, I always strove to get them back to 100% capacity to increase longevity by reducing sulfation and if we still had those, because of the extended taper on Pb batteries, the time to get back to 100% would have been 6 to 7 hours or so, with the last 3 hours just having the current dribble into the batteries at less than 30 amps or so on a 1280 Ah bank.
On our current Li bank, that taper period is reduced to 3 1/2 minutes. Note that the acceptance rate is totally flat - the bank absorbs what is put to it at around .3C with no fall off until the very end. The total re-charge time is 1 hour and 54 minutes, during which time house loads were drawing off some of the available charging current (we made bread, freezer and refer, etc.).
This also illustrates how difficult it is to adapt even sophisticated Pb charging equipment to Li chemistry, if what you want to do is ignore the charging cycle like you can when a Pb bank is connected to charging sources and those sources are managed by a smart regulator. If I had left the Li batteries charging, the voltage would have risen to the 28.8 bulk setting on both the Trace 4000 charger/inverter and Balmar 624 regulator. But to what end? The acceptance rate doesn't change materially after it drops below around 2.5% of capacity (in my case 15 amps), so having any absorption period where the bulk voltage is held over time is pretty pointless in terms of practical usage, and harmful in terms of just generating heat. After all, that charging current has to go somewhere, and once the Li battery is full it goes to heat, which will kill the battery. Partial recharging is just fine since Li batteries don't sulphate, and while some argue that if you don't fully re-charge memory effects will reduce total capacity, but I am not sure that is proven. If I wanted to get the batteries to 100% for drill, instead of the 95% or so achieved in the cycle described below, I would hold them at constant voltage for another 15 minutes or so, but again, there is no point in that, or at least one I am aware of.
In my mind, the perfect charging system for a Li battery would be able to automatically disconnect the Li bank from the charge sources at a user defined voltage, and automatically switch over to a running battery bank if underway or just start using the Li bank for loads if at anchor. I manage this manually now by using the BMS to disconnect the bank from charging or loads and use the Blue Seas ACR (automatic charging relay) to direct current to the Pb starter bank if we are underway once the Li batteries are full, or simply turn off the genset if we are recharging at anchor. In other words, all the existing charging equipment suited for Pb batteries can be used, but it has to be used differently to reflect the unique charging profile of this chemistry.
On our current Li bank, that taper period is reduced to 3 1/2 minutes. Note that the acceptance rate is totally flat - the bank absorbs what is put to it at around .3C with no fall off until the very end. The total re-charge time is 1 hour and 54 minutes, during which time house loads were drawing off some of the available charging current (we made bread, freezer and refer, etc.).
This also illustrates how difficult it is to adapt even sophisticated Pb charging equipment to Li chemistry, if what you want to do is ignore the charging cycle like you can when a Pb bank is connected to charging sources and those sources are managed by a smart regulator. If I had left the Li batteries charging, the voltage would have risen to the 28.8 bulk setting on both the Trace 4000 charger/inverter and Balmar 624 regulator. But to what end? The acceptance rate doesn't change materially after it drops below around 2.5% of capacity (in my case 15 amps), so having any absorption period where the bulk voltage is held over time is pretty pointless in terms of practical usage, and harmful in terms of just generating heat. After all, that charging current has to go somewhere, and once the Li battery is full it goes to heat, which will kill the battery. Partial recharging is just fine since Li batteries don't sulphate, and while some argue that if you don't fully re-charge memory effects will reduce total capacity, but I am not sure that is proven. If I wanted to get the batteries to 100% for drill, instead of the 95% or so achieved in the cycle described below, I would hold them at constant voltage for another 15 minutes or so, but again, there is no point in that, or at least one I am aware of.
In my mind, the perfect charging system for a Li battery would be able to automatically disconnect the Li bank from the charge sources at a user defined voltage, and automatically switch over to a running battery bank if underway or just start using the Li bank for loads if at anchor. I manage this manually now by using the BMS to disconnect the bank from charging or loads and use the Blue Seas ACR (automatic charging relay) to direct current to the Pb starter bank if we are underway once the Li batteries are full, or simply turn off the genset if we are recharging at anchor. In other words, all the existing charging equipment suited for Pb batteries can be used, but it has to be used differently to reflect the unique charging profile of this chemistry.